The word here could derive from Baec which is Old Dutch and Anglo-Saxon for "back". However, like many sayings, there are other suggestions as to the origin. The most likely of these is that, in the early 17th century "bacon" was thieves' slang for "escape". Alternatively, Brewer suggests it may mean the sides of home-killed bacon that every peasant family would have hanging up in the house; this would have been valuable property and if you or somebody else "saved your bacon" from fire or theft you would have had a narrow escape.